The present invention relates to control systems for multiple mode air conditioning units and, more particularly, to electronic mode control circuits of the type including momentarily actuated user mode selection switches and a volatile mode selection memory.
One form of air conditioning unit is sized for an individual room, and typically has local controls for both operational mode and temperature. A typical complement of operational modes is "High Fan Speed Heat", "Low Fan Speed Heat", "High Fan Speed Cool", "Low Fan Speed Cool", "Fan Only", and "OFF". From this listing of typical modes, it will be appreciated that the term "air conditioning" is employed herein in a broad sense to mean any form of unit which alters the characteristics of room air, for example, by either heating, cooling or both.
For selecting these various modes, at least two general forms of selector switching arrangements have heretofore been employed.
The first form of mode selector switching arrangement is an electromechanical switch. There are two common configurations of electromechanical switches: rotary detent-type switches; and interlocked mechanical push-button switches with exclusive selection capability arranged such that, when a desired selection push-button is depressed to complete a circuit to effect the particular function desired, all of the remaining push-buttons automatically pop up. Being mechanical in nature, an electromechanical type selection switch, once placed in a particular mode selection, in effect "remembers" that mode selection until moved to a different selection.
The second form of mode selector switching arrangement is electronic in nature, and is advantageously employed in combination with comprehensive electronic control systems which have recently come into use for such air conditioning units. It will be appreciated that electronic mode selection switching systems can be designed with a minimal number of moving parts, and are generally recognized as having greater long term reliability than electromechanical switches. Moreover, momentary "touch" switches of the deformable "membrane" type employed in electronic mode selector switching arrangements have a number of advantages in addition to reliability, including pleasing appearance and relatively low cost.
One example of an electronic mode selection circuit for an air conditioning system is described in commonly-assigned Pohl et al U.S. Pat. No. 4,287,939. This Pohl et al patent describes electronic momentary push-button and holding circuitry applied to the control of air conditioning units. To electronically provide a function similar to that of the electromechanical push-button configurations wherein only one push-button is actuated at a time, with actuation of any one push-button mechanically de-actuating all others, the electronic system employs momentary contact single pole switching elements, with electronic circuitry providing the holding function to keep a selected circuit or mode energized even after the user has released a push-button. Advantageously, the complexity required to mechanically interlock selector switches is eliminated. While a typical single pole switching element is a membrane "touch" switch, capacitive-type "touch" switches may as well be employed. In either event, it will be appreciated that the mode of operation is held or "remembered" by the electronic latching circuit, and not by mechanical elements of the switch itself.
A disadvantage, however, of such electronic latching circuitry relates to the manner in which they are powered, which is typically through a power supply circuit in turn supplied from a conventional AC branch circuit. Such a power source, while generally reliable, is nevertheless subject to interruption. In the event of a power interruption, the electronic circuitry typically unlatches. Thus, it "forgets" its mode of operation. Upon restoration of power, operation does not necessarily resume in the last mode actually selected by the user. Such latching circuitry is conventionally termed a "volatile" electronic memory because the stored information or data is lost when power is removed. While "non-volatile" electronic Random Access Memories (RAM's) are available, it is either impossible to change the stored information, as in the case of a Read Only Memory (ROM), or difficult, as in the case of an Electrically Programmable Read Only Memory (EPROM), or the memories are expensive and require relatively complex circuitry, as in the case of an Electrically Alterable Read Only Memory (EAROM).
It is common to include in electronic control circuits in general a "power on reset" circuit which forces the control circuit into a particular mode of operation when initially powered up, and also following power interruption. While such a power on reset circuit might be applied to an air conditioning control, most likely to reset the control to an "OFF" condition, there is no assurance that an appropriate mode of operation would occur upon resumption of operation. This is particularly a problem where the room user is away from the room for a period of time and, upon return following a power interruption and subsequent restoration, finds the room either excessively hot or excessively cold, depending upon the season.
It will be appreciated that typical DC power supplies include storage capacitors for the purpose of filtering rectified DC voltage and that these capacitors also prolong voltage supply to the mode memory circuit in the event of a power failure. In some cases, additional storage capacitors may be included in the circuit to maintain energization to selected circuit portions in the event of a power outage. However, such measures are only effective for a limited period of time. Such measures may well be effective to avoid loss of mode memory during short power interruptions, but not during prolonged power interruptions.
Another related approach is to connect a standby battery to maintain memory energization in the event of power failure. While this is an effective approach, it is relatively costly and, at some point the battery will need to be replaced, even if it is a rechargeble one. This is undesirable from a practical standpoint in an air conditioning unit supplied from a standard AC power source and expected to perform in a trouble-free and substantially maintenance-free manner for a number of years.